Semiconductor device having au-cu electrodes, and method of manufacturing semiconductor device

ABSTRACT

A method of manufacturing a biosensor semiconductor device in which copper electrodes at a major surface of the device are modified to form Au—Cu alloy electrodes. Such modification is effected by depositing a gold layer over the device, and then thermally treating the device to promote interdiffusion between the gold and the electrode copper. Alloyed gold-copper is removed from the surface of the device, leaving the exposed electrodes. The electrodes are better compatible with further processing into a biosensor device than is the case with conventional copper electrodes, and the process windows are wider than for gold capped copper electrodes. A biosensor semiconductor device having Au—Cu alloy electrodes is also disclosed.

This application claims the priority under 35 U.S.C. §119 of Europeanpatent application no. 11177457.6, filed on Aug. 12, 2011, the contentsof which are incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to semiconductor devices which are compatiblewith CMOS processing and methods of manufacturing the same. Inparticular it relates to semiconductor devices which are furtherprocessable to become biosensors and to methods of manufacturingbiosensors.

BACKGROUND OF THE INVENTION

Recently, bio-sensing devices have been developed which include asemiconductor device based on advanced CMOS processing techniques.Conventional biosensing devices typically rely on inert metals, such asgold or platinum, for their electrodes, in order to provide a high levelof compatibility with the often corrosive, or otherwise oxidising,environments in which such biosensors may be used, or which may berequired for downstream or subsequent processing of the semiconductordevice; however, there has been a recent move towards copper electrodes,in order to maximise the compatibility with standard CMOS processingsteps and equipment. However, copper is known to suffer from oxidisationand corrosion under conditions where biosensors are used.

A biosensor device is known from United States Patent Applicationpublication number US2009/184,002, which includes at least oneelectronic element having a metal electrode which may be made fromcopper or an alloy comprising copper. US2009/184,002 teaches that,whereas in general in biosensor devices noble metals such as gold silverand platinum are used it is preferable to use copper, since this iscompatible with advanced semiconductor processing. The copper readilyoxidises to form CuxOy, which may then be cleaned later in theprocessing, in particular in preparation for the application of a selfassembled monolayer, which may then act to protect the electrodes fromcorrosion and to couple biomolecules on top.

It would be desirable to provide a semiconductor device, and a method ofmanufacturing semiconductor device, which combines the advantages ofcopper-based electrode technology within the semiconductor device, andinert metal-electrodes for later processing as a biosensing device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductordevice, and a method of manufacturing a semiconductor device, which arecompatible with advanced CMOS processes and with further processing as abiosensor for biosensing applications.

According to the invention there is provided a method of manufacturing abiosensor semiconductor device having Au—Cu alloy electrodes at a majorsurface thereof, the method comprising: providing the semiconductordevice with Cu electrodes at the major surface and extending into thedevice; depositing a Au layer over the Cu electrodes; thermally treatingthe semiconductor device to alloy deposited Au and the Cu electrodes,thereby forming the Au—Cu alloy electrodes, and polishing thesemiconductor device to expose the Au—Cu alloy electrodes.

By providing, in the first place, copper electrodes, compatibility withCMOS processing, either standard CMOS processing or advanced CMOSprocessing, may be maintained. Moreover, by limiting the amount of freecopper at the surface (for instance, as would be the case by providing acapping layer of gold) the problems of oxidation of exposed copper,which occur with conventional biosensor devices having copperelectrodes, may be alleviated. Beneficially, according to embodiments,there is no requirement for a photolithography step to align the golddeposition with the electrodes. This is particularly beneficial inapplications in which the electrodes are microelectrodes such that thelateral dimensions of the electrodes are microns, or nano-electrodessuch that the dimensions of the electrodes are less than one micron.Thus the gold deposition need not be limited to being only over thecopper electrodes. That is to say, the gold layer may be deposited overand beyond the copper electrodes. It will be appreciated that theprovision of copper electrodes need not include their isolation and/orseparation; in particular, a copper layer may be deposited whichincludes the copper electrodes and a subsequent chemical mechanicalpolishing step may or may not be performed to remove copper from otherparts of the surface.

Furthermore, it will be appreciated that, in contrast to conventionalmetallisations involving deposition of gold over copper, no barrierlayer, such as Ti is required, since intermixing of the gold and copperis, in the present instance, necessary, rather than to be prevented by abarrier layer.

In embodiments, thermally treating the semiconductor device to alloydeposited Au and the Cu electrodes comprises completely alloying the Cuelectrodes. By consuming the copper into specific Au_(x)Cu_(y)compounds, the copper is “fixed” in the sense that it is no longer ableto migrate to the surface of the electrode or to corrode or form oxides.It will be appreciated that all or part of the deposited Au may bealloyed, depending on the specific conditions used. In embodiments, athickness of the deposited Au layer is at least 50 nm. Such a thicknessof the deposited layer of gold is typically sufficient to ensure thereis an excess of gold in order to allow all the copper to be consumed,that is to say fixed in specific Au_(x)Cu_(y) compounds. It will beappreciated that a thinner layer, for instance of only 10 nm, may bepossible; however, to achieve such a thin layer would generally requirean inconvenient thermal budget, such as an inappropriately longannealing time.

In embodiments, polishing the semiconductor device compriseschemical-mechanical polishing. Alternatively, other polishingtechniques, though generally less well-developed in the industry, may beused: for instance and without limitation, purely mechanical polishingor electropolishing, wherein the device is polished by the applicationof an electric field, inversely analogous to electroplating, may be usedto remove the unwanted metal.

In embodiments, thermally treating the semiconductor device comprisesheating the semiconductor device to a temperature which is within 50° ofeither 350° C. or 450° C. for a time which is within 15 minutes of onehour. These thermal treatments have been experimentally shown to providesufficient alloying of the gold and copper to result in a high level ofcorrosion resistance.

Since the concentration of copper in the alloy may vary across thedevice, with typically a higher level of copper above the electrodesthan elsewhere, it is preferable that the CMP step is carried out usinga slurry which balances the polishing rate of gold and copper. Forexample a copper-protecting agent may be added to a gold-CMP slurry inorder to reduce any acceleration of the polishing above the electrodesdue to a higher concentration of copper relative to elsewhere on thedevice.

In embodiments, the Cu electrodes comprise the final metal layer of amultilevel Damascene metallisation stack. Due to the multi-levelarrangement of a Damascene metallisation stack, migration of gold closeto the active CMOS areas of the semiconductor device, which may bedeleterious to the operation of the device, may be limited. Inembodiments, the semiconductor device comprises silicon or asilicon-containing material. Silicon-based material systems areparticularly suited for commercial biosensor applications, since theyare the most widely developed among semiconductor material systems.

The method may further comprise depositing a self-assembled-monolayer onthe exposed Au—Cu alloy electrodes.

It may further comprise a preceding step of separating the semiconductordevice from a wafer. Thus the invention is not limited to with the stateprocessing that may be carried out at a device level.

In embodiments, the electrodes have dimensions of no more than onemicrometre in lateral directions at the major surface of thesemiconductor. In embodiments, the biosensor is adapted for capacitivesensing, and the Au—Cu alloy electrodes comprise sensing electrodes.

According to another aspect of the invention, there is provided abiosensor semiconductor device having Au—Cu alloy electrodes at a majorsurface thereof and extending into the device. Au—Cu electrodes haveproven particularly beneficial in avoiding limiting corrosion of thesurface of the electrode, and thereby enhancing the adsorption orchemisorption of deposited materials such as self assembled monolayersfor use in association with or as bioreceptors in biosensingapplications.

In embodiments, the electrodes have a relatively Au-rich composition atthe major surface of the device, and a relatively Cu-rich compositionfurther from the major surface. Having a relatively Au-rich compositionat major surface helps to avoid corrosion at the surface; havingrelatively Cu-rich composition further from the major surface may assistin limiting the subsequent diffusion of gold towards the active areas ofthe device. In embodiments, the electrodes have dimensions of no morethan one micrometre in lateral directions at the major surface of thesemiconductor. Such embodiments are compatible with massively parallelcapacitive-based biosensing devices. In embodiments, the lateral spacingbetween adjacent electrodes is at least four times the width of adjacentelectrodes.

These and other aspects of the invention will be apparent from, andelucidated with reference to, the embodiments described hereinafter.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which

FIGS. 1( a) to 1(c) show schematic diagrams of a semiconductor device atthree stages of a method of providing a semiconductor device accordingto embodiments;

FIG. 2 shows a cross-section of part of a semiconductor device having aDamascene multilayer metallisation;

FIG. 3 shows a Au—Cu phase diagram;

FIG. 4 shows XRD analysis of Au—Cu layers before and after thermaltreatment; and

FIG. 5 shows a flow diagram according to embodiments.

It should be noted that the Figures are diagrammatic and not drawn toscale. Relative dimensions and proportions of parts of these Figureshave been shown exaggerated or reduced in size, for the sake of clarityand convenience in the drawings. The same reference signs are generallyused to refer to corresponding or similar feature in modified anddifferent embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

FIGS. 1( a) to 1(c) show schematic diagrams of a semiconductor device atthree stages of a method of providing a semiconductor device accordingto embodiments. FIG. 1( a) shows a schematic cross-section through asemiconductor device 100, being a major surface 105, and including thecopper electrodes 120 at the major surface and extending into thesemiconductor device. Within the body 110 of semiconductor device arevarious regions, layers and features associated with either standard oradvanced CMOS processing. Up to this stage of its manufacturer, thedevice is entirely compatible with standard or advanced CMOS processes.

FIG. 1( b) shows the semiconductor device 100, following the depositionof a gold layer 130 on the major surface of the device. As shown in FIG.1, the gold layer is deposited generally across the device, that is tosay there is no photolithography, or other masking the stage required tolimit the deposition of the gold to being only over the copperelectrodes. The gold layer 130 may be relatively thick, or the copperelectrodes relatively widely spaced, such that, compared to the volumeof copper in the copper electrodes, there is a higher volume of golddeposited on the device. Sputtering is a particularly convenientdeposition method for the gold layer 130. However, alternative types ofvacuum deposition such as evaporation may be used. It is noted that theinvention is not limited thereto, and yet other ways providing a goldlayer on the surface of the semiconductor device (for instance byelectroplating) may also be used.

In the next stage of the process, (not shown), the semiconductor deviceis subjected to a thermal treatment in order to promote the alloying ofthe gold layer 130 with a copper electrodes 120. Specific alloyingconditions may vary depending on the individual device and thethicknesses of the layers used; however it has been found that heatingthe semiconductor device to within 50° of 350° C. for between 45 min and1 hour 15 min, and particularly heating the semiconductor device to 350°C. for 1 hour, may provide a suitable degree of alloying of the copperand gold. In other experiments it has been found that heating thesemiconductor device to within 50° of 450° C. for between 45 min and 1hour 15 min, and in particular heating the semiconductor device to 450°C. to for 1 hour also provides a suitable amount of alloying of thecopper and gold, although in this case, the specific AuCu compounds andphases formed are different, to those formed at or around 350° C.

The skilled person will appreciate that the phase diagram of copper goldis nontrivial, and that diffusion of the two metals can occur even atroom temperature. A phase diagram of copper and gold is shown in FIG. 3.In general, the copper out-diffuses into the gold, and the goldin-diffuses into the copper. From the figure it may be seen that anAu—Cu alloy can include several specific compounds, for example AuCu₃,which can exist in two crystallographic phases, Au₃Cu which exists in asingle crystallographic phase, and AuCu which can also exist in twocrystallographic phases. Furthermore, in a relatively Au-richcomposition there may be solid solution of gold within the matrix of aspecific Au_(x)Cu_(y) compound; conversely in a Cu-rich environmentthere may be solid solution of copper within the matrix of a specificAu_(x)Cu_(y) compound.

Advantageously, all of the copper in the copper electrodes 120 may beconsumed forming one or more specific copper-gold Au_(x)Cu_(y) compoundssuch as Au₃Cu, AuCu₃, and AuCu. Even if all the copper is not consumed,the excess of gold which is available for the alloying process due tothe relatively thick deposited gold layer may provide that there is arelatively low or even negligible amount of free copper (that is to saycopper which is in a solid-state solution rather than in a specificalloy phases such as Au₃Cu) at the surface of the gold layer at the endof the thermal treatment. Typically, to provide good corrosionresistance, which is stable over time, a certain minimum ratio of goldto copper atoms the top of the electrode and no compositional gradientover the several atomic layers, such as for instance, the top 50 nm ofthe electrode, may be required. Furthermore, the excess of gold which isavailable for the alloying process due to the relatively thick depositedgold layer may provide that there is a relatively low or even negligibleamount of free copper (that is to say copper which is in a solidsolution rather than in a specific alloy phases such as Au₃Cu) at thetop of the electrodes—i.e. at the major surface 105 of the semiconductordevice.

Preferably the thermal treatment is carried out in a reducingatmosphere, such as foming gas, which has a composition of 5% H₂ and 95%N₂, in order to avoid or limit oxidation of any exposed copper. It hasfurther been found beneficial to ensure that the samples are allowed tocool to at most 100° C. whilst remaining in the reducing atmosphere.

At a following stage of processing, the semiconductor device isplanarised by CMP (chemical-mechanical polishing), to remove the excessgold from the semiconductor device. At the end of the CMP step, thesemiconductor device is as shown in FIG. 1( c). It will be appreciatedthat this figure is similar to that shown in FIG. 1( a); however, theoriginal copper electrodes 120 extending from the major surface into thesemiconductor device are now replaced by Au—Cu alloy electrodes 140. Itwill be appreciated that which specific alloy phase or phases of thealloyed copper electrodes is or are present is less critical than thatthere is a relatively low level of free copper at the surface 115 of theelectrode at the major surface of the semiconductor device. Preferably,the surface of the electrode comprises less than 2% free copper, andideally no free copper at all.

The skilled person will be aware that the efficiency and uniformity ofCMP depends heavily on the choice of polish compound or slurry used. Inthe present case, beneficially the entire surface is covered withAu-based material (gold, gold compounds or a solid-state solution ofcopper in gold), and the slurry should be chosen appropriately. Anexample of a suitable slurry is UltraSol A15 (supplied by Eminess)Advantageously, when compared to an un-alloyed gold capping layer, theCMP planarisation is much more uniform, and does not suffer to the sameextent from the effects of dishing which can occur with CMP of gold.“Dishing” can occur when a soft material such as gold is CMP polished,adjacent to a harder material, such as SiOx or other dielectricmaterials typically present at the surface of a semiconductor device.Uniform polishing is easier to achieve with harder materials such asAu—Cu alloys.

Experimentally, it has been found that, addition of a copper-protectingagent (such as BTA, that is 1,2,3-Benzotriazole) into a gold-CMP slurrysuch as UltraSol A15 has been found to improve the uniformity of the CMPprocess: since the Cu concentration over the electrodes is generallyhigher than that over the remainder of the device, it has surprisinglybeen observed that gold-slurries may have a higher polishing rate abovethe electrodes and over the remainder of the device. This suggests thatcopper dissolution is a significant mechanism in the gold CMP process;experimentally it has been verified that inclusion of acopper-protecting agent can significantly improve the planarisationuniformity.

The skilled person will readily appreciate that, since the CMP stepshould stop at the surface of the semiconductor device, a CMP slurryshould preferably be one which has as high selectivity againstdielectrics such as silicon nitride or silicon oxide.

Dishing is particularly problematic where there are large electrodes(such as bond pad electrodes). Towards the end of the CMP, since thegold at the bond-pad is removed much more easily than the relativelyhard material or materials (typically an oxide or other dielectricmaterial) at other positions on the surface of the semiconductor device,the gold is preferentially polished away, leading to a relatively lowsurface, or “dish”, in the bond pad area relative to the remainder ofthe surface. Whereas this effect is particularly significant for largerbond pads, it also can occur to a lesser extent at other electrodes,such as nanoelectrodes with lateral dimensions of less than a micron,which may typically be used, in a large array, for biosensorapplications.

If a gold capping layer were to be used absent a thermal alloyingtreatment, then in subsequent CMP, it has been observed experimentallythat the underlying copper of the original electrode may be exposed,particularly towards the central area of bond-pads. This may lead tofurther problems, such as the creation of an electrochemical cellbetween the gold and copper resulting in fast erosion and corrosion ofthe copper.

Furthermore, for CMP of gold-copper alloys, advantageously, water-basedslurries may be used rather than the nonconductive slurry such as IPA(iso-propyl-alcohol) containing cerium oxide particles which arerequired for CMP polishing of pure gold. Water-based slurries may beless expensive and more environmentally friendly compared with thosebased on other solvents such as IPA. Thus the process window for CMPaccording to embodiments is much wider than would be the case for puregold.

In other embodiments, other polishing techniques, such as purelymechanical polishing, may be used instead of chemical-mechanicalpolishing.

FIG. 4 shows experimental results for the alloying process. On thefigure is plotted, in arbitrary units, x-ray diffraction (XRD)measurements of the composition of the gold and copper electrodes,before heat treatment at curve 410, and after heat treatment, for twoand half hours at 250° C. and 350° C. at curves 420 and 430respectively, and after one hour at 450° C., at curve 440. As can beseen from the figure, the curves for the untreated electrodes, and theelectrodes heated to 250° C. for two and half hours both showsignificant peaks for elemental gold (at 450 and 452), and for elementalcopper (at peak 454). In contrast, in the curves 430 and 440 followingheat treatment at high temperatures, these peaks have almost completelydisappeared; instead, there is a noticeable peak at the position forAuCu in the case of curve 430, and for AuCu₃ in the case of curve 440.These experimental results demonstrate that elemental copper inparticular has been consumed in the alloying process. Since thegold-copper alloys are far less susceptible to corrosion, that is to sayoxidation, than elemental copper, the resulting electrode surface ismore stable and compatible with good adhesion of, for instance, selfassembled monolayers such as may be used in further processing of abiosensor device.

Further experimental analysis of the surface composition of theelectrodes following the alloying step and planarisation by CMP, havedemonstrated that after an alloying step of one hour at 350° C., thesurface contains 43±3 atomic percent of gold, and after an alloying stepof one hour at 450° C., the surface contains 35±1 atomic percent ofgold.

Experimentally, improved corrosion protection relative to bare copperelectrodes has been demonstrated by using corrosion tests in aphysiological buffer. In particular a 150 mM chloride containing salthas been applied, as well as 10 mM glycine solutions acidified to a pHlevel of 2: the CMP gold-copper alloy electrode reveals relatively lesscorrosion; in one case no visible damage was been observed by SEM after30 minutes in 10 mM glycine solution or after 4 hours in 150 mM chloridesolutions.

FIG. 5 shows a flow diagram 500 according to embodiments. Semiconductordevice 100 is provided, at step 510, with copper electrodes at its majorsurface and extending into the device. Up to this stage, the processingof the devices may be entirely conventional. Thereafter, at step 520,the gold layer is deposited over the device. The deposition of the goldis not limited to being over just over the copper electrodes; thus nophotolithography step involving accurate alignment to the electrodes isrequired. It should be noted that although the gold deposition is notlimited to being over the copper electrodes, it is not necessary thatthe gold is deposited over the entire device. Furthermore, thedeposition may be done at a device level, that is to say afterindividual devices have been separated from the processed semiconductorwafer; however, in general it will be the case that the gold depositionoccurs at the wafer level, that is to say before the wafer is separatedinto individual devices or dice.

Next, at step 530 the die or wafer is subjected to a thermal treatment,typically in a reducing atmosphere, in order to promote theinterdiffusion of the copper and gold to produce a gold-copper alloy.Whilst copper may diffuse laterally or up from the major surface, goldfrom the deposited gold layer diffuses into the copper electrode anddown or into the device away from the major surface. Due to the relativeexcess of gold, which typically results from the deposited gold layernot being constrained to being only over the copper electrodes, thecopper from the electrodes may be significantly or entirely consumed ortake up in the alloy, whereas the gold typically is not.

Thereafter, at step 540, the device, or the wafer in the case that thedepositions and alloying is carried out at a wafer level, is chemicalmechanically polished, in order to remove the remaining excess gold,which will typically have been at least partially converted to agold-copper alloy, from the device except at the electrodes. The alloyedgold-copper electrodes are thereby exposed.

In the above mentioned process flow, it is assumed that the biosensorsemiconductor device has been processed conventionally, up to the stagewhere the only exposed copper at the major surface is at the electrodes.That is to say, the fabrication of convention electrodes is complete.However, in other embodiments, the process may be slightly different: ina typical process to produce the top copper electrodes, such asDamascene, the metal, in this case copper, is deposited across thecomplete device, including into preformed partial vias. (By partial viasis meant vias which extend from the major surface into the device, butgenerally not completely through the device). In conventionalprocessing, the device is then planarised by CMP in order to removemetal from unwanted areas and retain metal only in the partial vias. Inembodiments, the copper layer is not planarised, rather, afterdepositing this layer, the gold layer 130 is deposited at step 520. Inother words, relative to the above-described process flow, step 510 ismodified in that it does not include any CMP planarisation of thecopper. The process then proceeds as described above, with thedifference that the gold interdiffuses into the copper not just at theelectrodes but across the whole device. To provide the preferred goldexcess, a relatively thicker layer of gold must be deposited, whencompared with the process flow described above. However, this modifiedprocess flow has the advantage that the interdiffusion between gold andcopper is much more uniform across device, which may result in a widerprocess window for the subsequent CMP stage 540.

Seen from one perspective, then, there is disclosed herein a method ofmanufacturing a biosensor semiconductor device, in which copperelectrodes at the major surface of the semiconductor devices aremodified to form Au—Cu alloy electrodes. The modification is effected bydepositing, typically by sputtering, a gold layer over the device, andthen thermally treating the device to promote interdiffusion between thegold and the electrode copper and to alloy them. The alloyed gold-copperis removed from the surface of the device typically by CMP, leaving theexposed electrodes. Since the gold-copper alloy is harder than gold, theCMP process window is wider than would be the case of pure gold;moreover, since the electrode copper has been converted to a gold-copperalloy, it is more corrosion resistant than a conventional copperelectrode. The electrodes are thus better compatible with furtherprocessing into a biosensor device than is the case with conventionalcopper electrodes, and the process windows are wider than for goldcapped copper electrodes. A biosensor semiconductor device having Au—Cualloy electrodes is also disclosed.

As will be appreciated by the skilled person, the term “biosensor” asused here in denotes a sensor which is capable of sensing biologicallyactive molecule or complex. Biologically active molecules are typicallylarge, organic molecules.

From reading the present disclosure, other variations and modificationswill be apparent to the skilled person. Such variations andmodifications may involve equivalent and other features which arealready known in the art of biosensing semiconductor devices, and whichmay be used instead of, or in addition to, features already describedherein.

Although the appended claims are directed to particular combinations offeatures, it should be understood that the scope of the disclosure ofthe present invention also includes any novel feature or any novelcombination of features disclosed herein either explicitly or implicitlyor any generalisation thereof, whether or not it relates to the sameinvention as presently claimed in any claim and whether or not itmitigates any or all of the same technical problems as does the presentinvention.

Features which are described in the context of separate embodiments mayalso be provided in combination in a single embodiment. Conversely,various features which are, for brevity, described in the context of asingle embodiment, may also be provided separately or in any suitablesub-combination.

The applicant hereby gives notice that new claims may be formulated tosuch features and/or combinations of such features during theprosecution of the present application or of any further applicationderived therefrom.

For the sake of completeness it is also stated that the term“comprising” does not exclude other elements or steps, the term “a” or“an” does not exclude a plurality, and reference signs in the claimsshall not be construed as limiting the scope of the claims.

1. A method of manufacturing a biosensor semiconductor device havingAu—Cu alloy electrodes at a major surface thereof, the methodcomprising: providing the semiconductor device with a plurality of Cuelectrodes at the major surface, said Cu electrodes extending into thedevice; depositing an Au layer over the Cu electrodes; thermallytreating the semiconductor device to alloy the deposited Au and the Cuelectrodes, thereby forming the Au—Cu alloy electrodes, and polishingthe semiconductor device to expose the Au—Cu alloy electrodes.
 2. Themethod of claim 1, wherein the step of thermally treating thesemiconductor device to alloy the deposited Au and the Cu electrodescomprises completely alloying the Cu electrodes.
 3. The method of claim1, wherein a thickness of the deposited Au layer is at least 50 nm. 4.The method of claim 1, wherein the step of polishing the semiconductordevice comprises chemical-mechanical polishing.
 5. The method of claim1, wherein the step of thermally treating the semiconductor devicecomprises heating the semiconductor device to a temperature which iswithin 50° C. of either 350° C. or 450° C. for a time which is within 15minutes of one hour.
 6. The method of claim 1, wherein the Cu electrodescomprise a final metal layer of a multilevel Damascene metallisationstack.
 7. The method of claim 1, wherein the semiconductor devicecomprises one of silicon and a silicon-containing material.
 8. Themethod of claim 1, further comprising at least one of depositing aself-assembled-monolayer on the exposed Au—Cu alloy electrodes, and apreceding step of separating the semiconductor device from a wafer. 9.The method of claim 1, wherein the electrodes have dimensions of no morethan one micrometre in lateral directions at the major surface of thesemiconductor.
 10. The method of claim 1, wherein the semiconductordevice is adapted for capacitive sensing, and the Au—Cu alloy electrodescomprise sensing electrodes.
 11. A biosensor semiconductor device havinga plurality of Au—Cu alloy electrodes at a major surface thereof andextending into the device.
 12. A biosensor semiconductor device as inclaim 11, wherein the electrodes have a relatively Au-rich compositionat the major surface of the device, and a relatively Cu-rich compositionfurther from the major surface.
 13. A biosensor semiconductor device asin claim 11, wherein the electrodes have dimensions of no more than onemicrometre in lateral directions at the major surface of thesemiconductor.
 14. A biosensor semiconductor device as in claim 11,wherein a lateral spacing between adjacent said electrodes is at leastfour times a width of adjacent said electrodes.
 15. A biosensorsemiconductor device as in claim 11, wherein the biosensor is adaptedfor capacitive sensing, and the Au—Cu alloy electrodes comprise sensingelectrodes.